Oxymethylene polymers represent an important class of engineering resins due to numerous favorable physical properties. For this reason, oxymethylene polymers have a wide range of commercial applications, for example, as parts for automobiles, as plumbing components and a variety of household and personal products.
It is oftentimes desirable to modify one or more of the inherently advantageous physical properties of oxymethylene polymers so as to meet the needs of specific end-use applications. Normally, to achieve such modified properties, oxymethylene polymers are usually blended with a variety of other resins and/or ingredients (e.g., impact modifying agents, flame retardants, light and heat stabilizers, fillers, and the like). Usually the blending of property-modifying agents with oxymethylene polymers is not without its own problems due principally to the highly crystalline nature of oxymethylene polymers which is evident in a low level of compatibility with other polymers.
Grafting of functional compounds or polymers (referred collectively to hereafter as functional compounds) onto an functional oxymethylene backbone with diisocyanate coupling agents would present an attractive alternative to blending so as to achieve block copolymers having the desired modified properties and/or to employ such graft copolymers as compatibilizing agents for compositions containing blends of oxymethylene polymer and a polymer with the same backbone as the functional polymer or a polymer which is compatible with the backbone of the functional polymer. However, with oxymethylene polymers, grafting is usually not possible due to the low level of polyacetal end group functionality --that is, since each oxymethylene molecule carries a maximum of two functional groups, e.g., hydroxyl end groups.
According to the present invention, however, grafting of functional compounds or polymers onto oxymethylene backbones with diisocyanate coupling agents is accomplished by increasing the reactive sites on the oxymethylene polymers. That is, the oxymethylene polymers employed in the present invention will exhibit increased functionality, in the form of reactive pendant hydroxyl groups. Hence, these oxymethylene polymers with increased hydroxyl functionality may be reacted with suitable functional compounds, diisocyanate coupling agents, and optionally, monofunctional end capping agents so as to obtain the graft copolymers of this invention.
The preferred oxymethylene polymer backbones onto which the functional compounds are grafted are essentially random copolymers containing oxymethylene units interspersed with higher oxyalkylene units having pendant hydroxyl groups. These pendant functional hydroxyl groups of the higher oxyalkylene units therefore provide reactive sites for the grafting of isocyanate compounds onto the oxymethylene backbone.
The novel graft copolymers of this invention may find usefulness as an engineering resins per se (i.e., as resins in which the functional compounds are chemically bound to the oxymethylene backbone), or as compatibilizing agents so as to compatibilize blends of oxymethylene polymers with other polymers having the same or similar backbones as the backbone of the functional compound or with other polymers that are compatible with the polymer backbone of the functional compound. For example, graft copolymers could be used to improve the mechanical and impact properties of blends of oxymethylene polymers with other appropriate polymers through improvement of interfacial adhesion between incompatible polymers.
The novel graft polymers may also find usefulness as modifiers and additives to oxymethylene polymers with improved effectiveness through improved compatibility.
Further aspects and advantages of this invention will become more clear after consideration is given to the following detailed description thereof .